Added ButtonFormType to allow the usage of icons in buttons

Hi,

i extended the button type to allow the usage of FontAwesome/Glyphicons in buttons generated by the form component..
Usage:

    class CertifierType extends AbstractType
    {
        public function buildForm(FormBuilderInterface $builder, array $options)
        {
            $builder

            ...

                ->add(
                    'save',
                    'submit',
                    [
                        'label' => 'certifier.form.save',
                        'icon'  => 'save'
                    ]
                );
        }

        ...

    }

results in something like

    <button class="btn" name="mcl_entrycertificate_webbundle_shipmenttype[save]" id="mcl_entrycertificate_webbundle_shipmenttype_save" type="submit">
        <i class="icon-save"></i>  Speichern
    </button>